Calendar of Physics Talks Vienna

Ultracold atoms are excellent systems to quantum-simulate the physics of ideal condensed-matter systems. A fundamental tool for this investigation is represented by optical lattices, i.e. periodic potentials generated by laser standing waves. Quantum phase transitions can be induced and investigated thanks to the possibility of accurately tuning the interactions between the particles, their mobility in the lattice, the amount of disorder and the system dimensionality. We will review some of the recent developments achieved at LENS in this field by studying ultracold quantum gases and mixtures in optical lattices.
We will focus on the possibility of introducing short-scale inhomogeneities in the lattice, which allows to study the interplay of interactions and disorder in the superfluid-insulator transition, one of the central problems of contemporary condensed-matter theory. We will discuss the observation of Anderson localization for a non-interacting Bose-Einstein condensate in a disordered optical lattice and the ongoing research to study the physics of localization in the presence of interactions between the atoms.
In order to characterize the properties of the different quantum phases novel diagnostic techniques have to be implemented. Recently, inelastic light scattering (Bragg spectroscopy) has allowed to study the excitations of 1D bosonic gases across the superfluid to Mott insulator transition. We will discuss this technique and the perspectives of using this tool to study the physics of disordered and strongly correlated 1D systems.

Date:

Mon, 04.05.2009

Time:

17:30

Duration:

60 min

Location:

Hörsaal des Atominstituts, Stadionallee 2, 1020 Wien

Contact:

Prof. Jörg Schmiedmayer

Vienna Theory Lunch Club - Is it always that electrons are unfriendly?

Speaker:

Catalina Petrascu (Frascati)

Abstract:

We believe that all processes in Nature are based on the fundamental principle relating spin and symmetry, sometimes known as the Pauli Exclusion Principle (PEP). Star evolution, chemical processes, atoms, protons and neutrons are just few examples of PEP at work. WE are examples of PEP at work!It is a well known fact that electrons are “selfish”, they do not like to share the same quantum state; i.e. in any object where electrons are present, let’s take an atom, each one has a unique set of quantum numbers – no other electron share it! Such a matter of fact is not a miracle – it has its deep roots in the concept of identical particles, which, together with the more elusive concept of spin, generates in the quantum mechanics the so-called spin-statistics relation, assigning always to the fermions (particles with half integer spin, as electrons) an antisymmetric wave-function (obeying the PEP principle), while to the more friendly bosons (particles with integer spin, as the photons) a symmetric one -they can and sometime do share the same quantum state.
Now, why to wonder about the PEP validity? Are we not happy it works? Otherwise we would not exist after all….
The quest for PEP validity was actually initiated by Pauli himself in 1945, when, in his Nobel lecture (prize received for the PEP discovery!) he said: “... Already in my original paper I stressed the circumstance that I was unable to give a logical reason for the exclusion principle or to deduce it from more general assumptions. I had always the feeling and I still have it today, that this is a deficiency. ... The impression that the shadow of some incompleteness [falls] here on the bright light of success of the new quantum mechanics seems to me unavoidable”
Since then, in spite of more than 50 years of physics, we are still wondering about PEP validity, since we were not yet able to answer to Pauli – we did not yet find the logical reason beyond PEP! On the contrary, isolated voices are even claiming that sometimes it might not even hold…blasphemy? So, we are entitled and motivated to look for its violation, and such experiments are done following various, ingenious, methods. We shall present the origin of PEP and the VIP experiment, the most ambitious search for its violation by electrons, as well as a discussion about possible outcomes – Nobel or….IgNobel (big difference, isn’t it?). Because (citing O. Greenberg):“Hopefully either violation will be found experimentally or our theoretical efforts will lead to understanding of why only bose and fermi statistics occur in Nature”.Overview lunch club

Metal nano-particles sustain so-called localized surface plasmon (LSP) modes, which are coherent electronic excitations coupled to light in the visible to near infrared spectral range. If excited resonantly, LSP modes lead to optical near field enhancement in a sub-wavelength spatial domain around the metal nano-particle, and related to this are surface enhanced effects, like surface enhanced Raman scattering or -fluorescence.
Regular arrays of quasi-identical, lithographically fabricated metal nano-particles are a flexible platform for systematic research on surface enhanced effects. I will present some recent results, which demonstrate the validity of the electromagnetic SERS model beyond the |E|^4 approximation and show, how in turn surface enhanced effects can be utilized to map the optical near field of the metal nano-particles.